Adaptive cruise control (ACC) for driving comfort is, for example, a method of automatically controlling the speed of a vehicle. Specifically, when there are no leading vehicles, the adaptive cruise control works to set the vehicle's speed to a desired constant speed determined by, for example, the driver.
When a leading vehicle is encountered, the adaptive cruise control works to alter the vehicle speed to maintain a desired interval with respect to the leading vehicle while following the leading vehicle.
In addition, pre crash safety (PCS) for driving safety is, for example, a method of providing braking assist and seatbelt retraction, and prompting the driver to take action to avoid a collision and reduce damage when it is determined that the vehicle will collide with a leading vehicle.
A conventional radar device disclosed in U.S. Pat. No. 5,717,399 corresponding to Japanese Patent Application Publication No. H08-146131 includes:                a first radar module to measure a first distance between a subject vehicle in which the radar device is installed and a remote leading vehicle in front of the subject vehicle for execution of the ACC; and        a second radar module to measure a second distance between the subject vehicle and an adjacent obstacle in front of the subject vehicle for execution of the PCS.        
In the conventional radar device disclosed in the U.S. Pat. No. 5,717,399, if a failure occurs in, for example, the first radar module, the ACC based on the first distance measured by the first module cannot be executed.
Similarly, in the conventional radar device disclosed in the U.S. Pat. No. 5,717,399, if a failure occurs in, for example, the second radar module, the PCS cannot be executed.
Specifically, in the conventional radar device disclosed in the U.S. Pat. No. 5,717,399, a malfunction in either the first radar module or the second radar module may reduce corresponding one of driving comfort and driving safety.